High capacity networks are frequently constructed using optical links. Each optical link may include one or more optical fibers through which optical signals are transmitted. The light transmitted through the fibers is controlled in such a way as to communicate desired information.
Orthogonal Frequency Division Multiplexing (OFDM) is a frequency division multiplexing scheme that has recently been applied to optical networks to obtain high bandwidth optical communication. In general, an OFDM signal is composed of a number of closely spaced and partially overlapping subcarriers. The data is divided into several parallel data streams or channels, one for each subcarrier. Each subcarrier may be modulated with a conventional modulation scheme at a relatively low symbol rate, thereby maintaining the total data rate at a level comparable to single-carrier modulation schemes in the same bandwidth.
FIG. 1 is a diagram illustrating a conventional receiver 100 for demodulating optical OFDM signals using a coherent detection technique. Receiver 100 may implement coherent signal detection, meaning that the carrier is removed from the received signal and a local oscillator is used to regenerate the carrier at the receiver.
As shown in FIG. 1, receiver 100 includes a local oscillator 110, a 2×2 optical coupler 115, a balanced detector 120 (which includes matched photodiodes 120 and a differential amplifier 125). Input signal 105 and the output of local oscillator 110 are input to 2×2 optical coupler 115. The outputs of optical coupler 115 are transmitted to balanced detector 120.
Local oscillator 110 may include a laser that generates an optical signal that is matched to the optical signal used by the transmitter (i.e., the transmitter of the optical signal received by receiver 100). Local oscillator 110 is used in coherent detection because the carrier in input signal 105 is removed by the transmitter before transmitting the signal over the optical channel. In practice the frequency of local oscillator 110 may not be perfectly matched to the oscillator used for transmission and additionally, local oscillator 110 may include a non-zero line width that introduces phase noise into the system.
Optical coupler 115 may act to optically mix (interfere) its two input optical signals to generate output signals in which the signal from local oscillator 110 is mixed with input signal 105. One arm contains the sum of the two optical signals and one arm contains the difference of the two optical signals. Matched photodiodes 121 of balanced detector 120 may receive its two input optical signals and produce voltages proportional to the square of the electric field of the input optical signals. Differential amplifier 125 subtracts the signals output from matched photodiodes 120 to obtain electrical signal 130, which can be processed to recover the transmitted (desired) information. More particularly, signal 130 may subsequently be converted to the digital domain by an analog-to-digital converter and then processed using signal processing techniques to obtain the desired information.
Advantages of receiver 100 include: (1) that the OFDM signal is optically power efficient on the transmission line since it only contains the data carrying subcarriers, and (2) that groups of subcarriers can be closely spaced to local oscillator 100 since subcarrier mixing products are suppressed by balanced detector 120. The disadvantages of receiver 100, however, include: (1) that local oscillator is 110 required at the receiver, 2) both local oscillator 110 and the laser at the transmitter should have narrow line widths since OFDM is sensitive to phase noise, and 3) significant processing effort and bandwidth in the form of pilot tones may be required for phase estimation.
FIG. 2 is a diagram illustrating another conventional receiver 200 for demodulating optical OFDM signals. Receiver 200 may implement direct signal detection, meaning that the carrier is transmitted along with the subcarriers.
Receiver 200 includes a single ended detector 220 that converts its input optical signal into an electrical signal. The advantages of direct detection include: (1) a relatively simple configuration of receiver 200 and (2) tolerance to high line width lasers since the phase of the laser is in the carrier. The disadvantages of receiver 200, however, can include: (1) optical power efficiency is halved since typically the carrier contains the same power as all of the subcarriers combined, and (2) spectral efficiency is reduced relative to coherent detection because detector 220 generates subcarrier mixing products that require a group of subcarriers to be sufficiently separated from the carrier so that the mixing products do not interfere with the subcarriers.
FIG. 3 is a diagram illustrating another conventional receiver 300 for demodulating optical OFDM signals. Receiver 300 is similar to receiver 200 in that both implement direct detection. Additionally, receiver 300 includes a spectral filter.
More particularly, as shown in FIG. 3, receiver 300 includes a spectral filter 315 in front of a single ended detector 320. Spectral filters selectively transmit optical signals according to wavelength. Spectral filter 315 may particularly operate to attenuate the signal subcarriers with respect to the carrier. The effect is to reduce the subcarrier mixing products generated by detector 320 as compared to the desired products between the carrier and the subcarriers.
The advantages of receiver 300 include: (1) tolerance to high line width lasers, 2) spectral efficiency since the reduction of the subcarrier mixing products allows the subcarriers to be moved closer to the carrier, and 3) optical power efficiency in the line since the carrier power can be reduced to a fraction of the total subcarrier power. The disadvantages of receiver 300, however, include: (1) an optical amplifier may be required at the receiver to boost the line's reduced carrier to a high optical power; (2) the attenuation of the subcarriers from their amplified levels can mean power is wasted in the amplifier; (3) subcarrier mixing products are reduced, but not completely suppressed; (4) the carrier noise products can be high relative to the subcarriers, requiring further RF spectral filtering; and (5) if the signal is not amplified enough, thermal noise in detector 320 can limit the signal-to-noise ratio (SNR).
The conventional OFDM receivers described with respect to FIGS. 1-3 each has advantages and disadvantages. It can be appreciated that, when using OFDM, it is desirable to use the most bandwidth maximizing and/or efficient receiver as possible.